1. Field of the Invention
The invention relates to molding machines, and particularly to the machine nozzle attached to the barrel of the heated injection cylinder and which engages the platen or manifold, delivering an intermittent flow of plastic as controlled by the machine cycle.
2. Description of the Prior Art
It is known to provide temperature control equipment for the machine barrel of molding equipment, this control means being rather elaborate and expensive. Temperature control means are also known for the sprue bushing and mold. However, in the past, attention has not been paid to the part which connects these two components of molding equipment, namely, the machine nozzle. Presently, machine nozzles are constructed with either external heating bands surrounding the nozzle body or a plurality of cartridge heaters which are inserted in axial bores in the body. Manual rheostats are conventionally provided for controlling the temperature by means of these heaters.
Aside from the obvious disadvantage of manually adjustable equipment, external heater bands and cartridge heaters have not been able to provide the required uniform distribution of heat in the machine nozzle. In the case of external heater bands, the fact that they do not extend along the entire shell housing of the machine nozzle creates cold spots at certain locations on the nozzle. This causes "splay" which are milky streaks created by the partial setting of some of the plastic being injected. In the case of cartridge heaters, since they are at circumferentially spaced locations around the body, portions of the plastic conduit between these locations are sometimes insufficiently heated, causing similar problems.
Other disadvantages of previous machine nozzles with the above-described heating means include, in some cases, short life of the heaters where they are of relatively small size for the amount of heat required. Moreover, cartridge heaters are relatively difficult to remove when replacement is required since they sometimes get stuck in the bores. In the case of machine nozzles with external heaters, another disadvantage is the possibility of leakage spray of molten plastic from between the nose of the machine nozzle and the manifold, causing damage to the external heater.
Applicant is aware of co-pending patent application Ser. No. 870,849 filed Jan. 19, 1978, by David M. Reitan and assigned to the assignee of the present application. This co-pending application shows a sprue bushing which bears some similarity to the machine nozzle of the present invention. However, several important structural features of the present invention are not found in this co-pending application. Moreover, the purpose and function of sprue bushings are quite different than those of machine nozzles. In this sense, therefore, the above-identified co-pending application is not considered to be pertinent to the present invention.